In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility, and large-area coverage. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the short-wavelength infrared (1-2 um) for telecommunications (1300-1600 nm), nightime and low-light imaging (1000 nm and beyond), biological imaging (800 nm and 1100 nm and 1300 nm transparent tissue windows), thermal photovoltaics (>1900 nm), and solar cells (800-2000 nm). Photodetecting and photovoltaic polymer devices have yet to demonstrate significant sensitivity beyond ˜800 nm.
Organic/nanocrystal composites have been demonstrated to enable a number of important optoelectronic devices operating in the visible region. In the infrared, electroluminescence has been demonstrated from such materials. Prior to the results presented herein, there has been no demonstration of a short-wavelength infrared photovoltaic effect from such a material system.
Control of organic-inorganic interfaces on the nanoscale is of critical importance in organic electronics, and in particular in photovoltaic devices based on inorganic quantum dots embedded in a semiconducting polymer matrix. In these systems, rapid and efficient charge separation is needed for subsequent separate transport and extraction of electrons and holes. Organic ligands passivating the surfaces of nanocrystals are needed to enable solution-processing without aggregation, yet these ligands are typically insulating and thus impede charge transfer between the nanocrystal and polymer. Moderate success has been achieved in conjugated polymer/inorganic nanocrystal composite-based solar cells active in the visible region, and these hold the promise for fabrication of large area photovoltaics on flexible substrates using low-cost processing methods such as solution spin coating. However, approximately 50% of solar energy reaching the Earth's surface lies in the visible region, and the remainder in the infrared (IR) region beyond 700 nm. It is therefore of great interest to develop IR sensitive devices, ultimately to enable harvesting of the full solar spectrum.
Infrared photoconductive and photovoltaic devices based on the solution-processable PbS quantum dot/MEH-PPV materials system have recently been reported. These first reports exhibited promising efficiencies meriting further optimization. Many factors can affect photovoltaic device performance, such as the effectiveness of charge separation and the magnitude of charge mobility, as well as the efficiency of charge collection. It is fundamentally important to understand these processes and to increase the effectiveness of these processes in the device in order to optimize performance.